We report whole-rock chemical compositions and the K-Ar age of obsidian gravel in a Miocene conglomerate in the Mikuni coastal area, Fukui prefecture, central Japan. The obsidian gravel samples are generally aphyric, and some contain crystallites. The estimated K-Ar age of the obsidian gravel sample is 13 Ma, within the age range of Miocene volcanic rocks (basalt to rhyolite) present in the Mikuni coastal area (14-12 Ma). The obsidian gravel samples plot at the SiO2-rich end of the Miocene volcanic rocks when comparing major elements versus SiO2. The MORB-normalized trace-element pattern and K-Ar age of the obsidian gravel are similar to those of the Miocene volcanic rocks around the Mikuni coastal area. These observations indicate that the origin of the rhyolite magma that produced this obsidian is associated with that of the Miocene volcanic rocks in the Mikuni coastal area. The rhyolitic magmatic activity occurred synchronously with the formation of these Miocene volcanic rocks, after the opening of the Japan Sea.